The present invention relates, in general, to semiconductor devices, and more particularly, to a novel metal oxide semiconductor field effect transistor.
In the past, the semiconductor industry has utilized a variety of structures to improve the breakdown voltage of power metal oxide semiconductor field effect transistors (MOSFETs). One particular implementation involves forming a lightly doped epitaxial drift layer on a heavily doped substrate. A second lightly doped layer is formed on the lightly doped drift layer. Trenches are etched completely through the second lightly doped layer and into the drift layer. A gate is created by forming an oxide on the sidewalls of the trenches and filling the trenches with gate material. Because the gate is adjacent the second lightly doped layer and the drift layer, the channel of the MOSFET includes both the second lightly doped layer and a portion of the drift layer. A source region is formed on the second lightly doped layer. One example of such a MOSFET is described in U.S. Pat. No. 5,323,040 issued to Baliga et al. on Jun. 21, 1994.
One problem with this prior implementation is the gate-to-drain breakdown voltage. Breakdown generally occurs near the corners of the oxide that is lining the trench. Consequently, the gate-to-drain breakdown voltage of the MOSFET is not as great as desired.
Accordingly, it is desirable to have a power MOSFET that does not have breakdown near the corners of the gate oxide, and that has a large gate-to-drain breakdown voltage.